This chapter introduces a robust mathematical formalism for the definition of deliberative agents implemented using a case-based reasoning system. The concept behind deliberative agents is introduced and the case-based reasoning model is described using this analytical formalism. Variational calculus is introduced in this chapter to facilitate to the agents the planning and replanning of their intentions in execution time, so they can react to environmental changes in real time. A variational calculus based planner for constructing deliberative agents is the presented and compared with other planners. Reflecting the continuous development in the tourism industry as it adapts to new technology, the chapter includes the formalisation of an agent developed to assist potential tourists in the organisation of their holidays and to enable them to modify their schedules on the move using wireless communication systems.